3/8-18 NPT Thread Dimensions and Tap Drill
3/8 NPT, written in full as 3/8-18 NPT, is the American taper pipe thread for nominal 3/8 inch pipe: 18 threads per inch cut on a 1 in 16 taper, on pipe with an outside diameter of 0.675 in (17.14 mm). The nominal size names the pipe and is not a dimension you can measure on the thread.
3/8 NPT is used for air lines, fuel fittings and small hydraulic connections.
- Tap drill37/64"0.5781 in, 14.68 mm
- Threads per inch18pitch 1.411 mm
- Pipe outside diameter0.675 in17.14 mm
- Pitch Ø at hand-tight plane0.62701 in15.926 mm
- Hand-tight engagement0.240 in4.32 threads
- Effective thread length0.4078 in7.34 threads
Where the diameters are measured
A taper thread has no single pitch diameter. ASME B1.20.1 defines it at the end of the pipe (E0), at the plane where a fitting stops when screwed on by hand (E1), and at the end of the effective thread (E2). Each step of 1 in along the axis adds 1/16 in to the diameter.
3/8-18 NPT dimensions
| Dimension | in | mm |
|---|---|---|
| Pipe outside diameter D | 0.6750 | 17.145 |
| Pitch P | 0.05556 | 1.411 |
| Pitch diameter at pipe end E0 | 0.61201 | 15.545 |
| Pitch diameter at hand-tight plane E1 | 0.62701 | 15.926 |
| Pitch diameter at end of effective thread E2 | 0.63750 | 16.192 |
| Hand-tight engagement L1 | 0.2400 | 6.096 |
| Effective thread length L2 | 0.4078 | 10.358 |
| Minor diameter at pipe end K0 | 0.5676 | 14.417 |
| Thread height h (0.8 × P) | 0.04444 | 1.129 |
| Property | Value |
|---|---|
| Threads per inch | 18 |
| Flank angle | 60° (30° each side) |
| Taper | 1 in 16 on diameter (3/4 in per foot) |
| Taper angle to the axis | 1° 47′ (1.7899°) |
| Threads engaged by hand | 4.32 |
| Wrench make-up | 3 threads |
Basic dimensions from ASME B1.20.1 Table 2. Millimetre values are conversions.
3/8 NPT tap drill size
Tap drill for 3/8 NPT: 37/64" (0.5781 in, 14.68 mm)
This is the size printed on standard tap drill charts. ASME B1.20.1 suggests the slightly smaller 9/16" (0.5625 in) for a fuller thread.
| Method | Drill | in | mm | Against K0 (in) |
|---|---|---|---|---|
| Standard tap drill chartusual choice | 37/64" | 0.5781 | 14.68 | +0.0105 |
| Drill only, no reaming (ASME B1.20.1) | 9/16" | 0.5625 | 14.29 | -0.0051 |
| Drill, then taper ream (ASME B1.20.1) | 9/16" | 0.5625 | 14.29 | -0.0051 |
The hole for a taper tap is a compromise, because a straight drilled hole meets a tapered thread. The minor diameter of the thread is 0.5676 in at the small end (K0) and grows by 1/16 in per inch of depth. The chart size of 37/64" is 0.0156 in larger than the 9/16" the standard suggests: it taps more easily but leaves shallower threads at the bottom of the hole. Use the smaller drill where the joint has to hold high pressure or the material is soft. For the fullest thread, drill with the reaming size and follow with a 1 in 16 taper pipe reamer.
ASME B1.20.1 notes that its suggested drills do not guarantee fully formed threads over the whole hand-tight length.
How far 3/8 NPT screws in
A 3/8 NPT fitting turns in by hand for 0.240 in (6.1 mm), about 4.3 threads, before the flanks meet. The standard then allows 3 more threads of wrench make-up, for a total engagement of about 0.407 in (10.3 mm). Tolerance on the hand-tight position is one thread either way, so a joint that stops a turn early or late is still within the standard.
NPT threads do not seal by themselves. The crests and roots leave a spiral gap, so ASME B1.20.1 calls for a sealant (PTFE tape or pipe compound) where the joint must be pressure-tight. Dryseal NPTF threads to ASME B1.20.3 have the same pitch and taper but controlled crests and roots that seal without it.
3/8 NPT against G3/8 BSP
The British Standard Pipe thread of the same nominal size has a different pitch (19 against 18 threads per inch), a 55° flank angle instead of 60° and a major diameter of 16.662 mm against 17.145 mm. The two do not screw together properly. Use an adapter.
| Thread | TPI | Major Ø (mm) | Major Ø (in) | Flank angle | Taper |
|---|---|---|---|---|---|
| 3/8-18 NPT (this thread) | 18 | 17.145 | 0.6750 | 60° | 1 in 16 |
| G3/8 (BSPP) | 19 | 16.662 | 0.6560 | 55° | parallel |
Related sizes and tools
Sources: ASME B1.20.1 (dimensions, Table 2; suggested drills, Appendix), Machinery’s Handbook and tap makers’ charts (shop tap drill sizes). How this site checks its data. Last reviewed 29 September 2026.